The basic formula for credit card APR

APR divided by 365, multiplied by your balance, multiplied by the number of days in your billing cycle — that is the interest charge you will see on your next statement. The math is straightforward once you know which balance the card issuer is using, because that is where most people get confused.

Here is a concrete example. Say you have a card with 18% APR and a $2,000 balance. Divide 18 by 365 to get your daily rate: 0.0493%. Multiply that by $2,000 to get the daily interest: $0.99. If your billing cycle is 30 days, multiply $0.99 by 30 to get $29.70 in interest charges for that cycle.

The catch is that card issuers do not charge interest on a single fixed balance. They use one of several methods to calculate which balance they charge interest on — the most common being the average daily balance method, which adds up your balance on each day of the cycle and divides by the number of days. If you paid down half your balance mid-cycle, the interest reflects that.

Key Takeaways

  • The daily periodic rate is your APR divided by 365, and that rate multiplies your balance each day to create the interest charge.
  • Most cards use the average daily balance method, which means paying down your balance mid-cycle reduces the interest you owe that month.
  • Your card's terms document (called the Schumer Box or the pricing and terms table) lists which calculation method the issuer uses.
  • Interest accrues during your grace period only if you carried a balance from the previous month; new purchases usually have no grace period once you carry a balance.
  • The stated APR assumes a full year; shorter time periods require you to divide the annual rate proportionally.

Where to find which calculation method your card uses

Your card issuer must disclose the balance calculation method in the pricing and terms table, usually on the back of your disclosure documents or in the online account portal under "terms" or "pricing information." The table will say something like "average daily balance (excluding new purchases)" or "previous balance method" or "two-cycle balance method."

The average daily balance method is by far the most common. It works like this: the issuer adds your balance at the end of each day in the billing cycle, then divides by the number of days. If you started with $2,000, paid $500 on day 15, and made no other changes, your average daily balance would be ($2,000 × 14 days) + ($1,500 × 16 days) divided by 30 days, which equals $1,733.33. That is the balance the interest calculation uses.

The previous balance method is less common and less favorable to you. It charges interest on whatever your balance was at the end of the previous cycle, regardless of what you paid this cycle. The two-cycle method, now rare, charges interest on the average of this cycle and the last cycle — this one is almost always worse for the cardholder.

How grace periods affect the APR calculation

If you pay your full statement balance by the due date, you owe no interest — the grace period protects you. But the grace period only applies to new purchases. If you carried a balance from the previous month, interest starts accruing on day one of the new cycle, even if you make no new charges.

This matters for the calculation because it changes when the interest clock starts. On a new purchase with a grace period, the issuer does not charge interest during those 21 to 25 days (the length varies by card). On a carried balance, interest accrues immediately at the daily periodic rate. If you have both a carried balance and new purchases in the same cycle, the new purchases accrue interest only after the grace period ends.

Some cards offer a 0% introductory APR for a set period — typically 6 to 21 months on purchases or balance transfers. During that period, the calculation is the same, but the rate is 0%, so the interest charge is $0. Once the intro period ends, the regular APR kicks in and the full calculation applies.

Calculating interest on a balance transfer

Balance transfers often have a different APR than purchases, and sometimes a different grace period (usually none). If you transfer a balance, the interest calculation starts immediately, using the balance transfer APR, not the purchase APR.

Say you transfer $5,000 at 0% for 12 months, then 18% after. During those 12 months, your interest charge is $0 each cycle. On month 13, the 18% APR kicks in. If you still owe $5,000, your daily rate becomes 0.0493%, your daily interest becomes $2.47, and over a 30-day cycle you owe $74.10 in interest.

The balance transfer APR applies only to the transferred amount. Any new purchases on the card use the purchase APR. If you transfer $5,000 at 0% and then charge $500 in new purchases, the $5,000 uses the transfer rate and the $500 uses the purchase rate (and the purchase rate may have a grace period while the transfer does not).

What happens when you make multiple transactions in one cycle

The average daily balance method handles multiple transactions by tracking your balance each day. If you charge $1,000 on day 5, pay $500 on day 12, and charge $300 on day 20, the issuer adds up the balance for each day and divides by the cycle length.

This is why paying down your balance mid-cycle reduces your interest charge — the lower balance applies to the remaining days of the cycle. If you pay $500 on day 12 of a 30-day cycle, that $500 reduction counts for 18 days, which meaningfully lowers your average daily balance and your interest charge.

The order of transactions matters only for the daily balance calculation. Payments typically post the same day or the next business day, so a payment on day 12 usually reduces your balance starting day 12 or day 13. Charges post immediately or within one business day. Your card's terms document specifies the posting timeline.

Calculating APR for time periods shorter than a year

APR is always stated as an annual rate, but you may want to know the interest cost for a specific number of days or months. The formula is the same, but you adjust for the time period.

For a 30-day cycle: (APR ÷ 365) × balance × 30. For a 60-day period: (APR ÷ 365) × balance × 60. For three months: (APR ÷ 365) × balance × 90. The daily rate (APR ÷ 365) stays constant; only the number of days changes.

If you want to know the monthly interest cost on a $3,000 balance at 20% APR, the math is (20 ÷ 365) × $3,000 × 30 = $49.32 per month. Over 12 months, that would be $591.78 in interest, which is close to 20% of $3,000 ($600) — the small difference is because 365 days is not exactly 12 months of 30 days.

How to use your card's online tools to check the calculation

Most card issuers show you the interest charge on your statement before you pay it. The statement lists the previous balance, payments, new charges, fees, and the interest charge for that cycle. You can reverse-engineer the calculation from that information to verify it matches the formula.

Some issuers also provide an interest calculator in the online account portal or mobile app. You enter your balance and the calculator shows you the interest charge for that cycle or for a hypothetical balance. This is useful for planning — if you are considering carrying a balance, you can see what the interest cost will be before you commit.

Your statement also shows the daily periodic rate, which is the APR divided by 365. If your APR is 18%, the daily periodic rate should be 0.0493%. You can use that number directly in the formula: daily periodic rate × balance × number of days = interest charge.

Frequently Asked Questions

Does the APR change during my billing cycle?

No. The APR that applies to your cycle is set at the start of the cycle and does not change mid-cycle, even if the card issuer raises rates for new cardholders. However, the issuer can raise your APR with 45 days' notice, and the new rate applies to the next billing cycle after the notice period ends. Check your statement or online account for any rate change notices.

What is the difference between APR and the interest charge on my statement?

APR is the annual rate. The interest charge is what you actually owe for one billing cycle. If your APR is 18% and your average daily balance is $2,000, your interest charge for a 30-day cycle is about $29.70, not 18% of $2,000. The APR is annualized; the interest charge is the daily rate applied to your specific balance and cycle length.

If I pay my balance in full, do I owe any interest?

Not on purchases, if you pay by the due date. The grace period means you owe no interest on new purchases as long as you pay the full statement balance. However, if you carried a balance from the previous month, interest accrues on that carried balance even if you pay everything off this cycle. Only new purchases get the grace period protection.

How does a 0% APR offer change the calculation?

The calculation stays the same, but the rate is 0%, so the interest charge is $0. Once the 0% period ends, the regular APR applies and interest accrues normally. Mark the end date of the 0% period in your calendar so you are not surprised by interest charges when it expires.

Can I calculate interest on a partial month?

Yes. Use the formula (APR ÷ 365) × balance × number of days. If you want to know the interest on $1,000 at 15% APR for 15 days, the math is (15 ÷ 365) × $1,000 × 15 = $6.16. This is useful if you are planning to pay off a balance on a specific date and want to know the total cost.